1. Field of the Invention
The present invention relates generally to semiconductor or lasers and more specifically to a modulator-integrated semiconductor laser.
2. Description of the Related Art
For optical communications systems, semiconductor lasers essential elements and their ability to modulate the laser beam with information signals is an important factor to achieve high speed transmission. One approach to achieving this goal is to reduce the device capacitance. In conventional semiconductor lasers, a light waveguide including the active layer of the laser is buried in an InP embedding layer and a contact layer such as InGaAs or InGaAsP is formed on the embedding layer. Through the contact layer, the embedding layer are selectively implanted with proton to create high resistance regions on the opposite side portions of the active layer, As a result of the carrier confinement effect of the high resistance regions, injection current is efficiently utilized by the active layer. An electrode is then formed on the contact layer. Although the contact layer is formed of the material known to exhibit inability to become highly resistive when subjected to a beam of protons, the electrode that extends over the full area of the device contributes to an increase in the device capacitance, which prevents the laser from operating at a speed of 10 Gb/s or higher.
Another approach to reducing device capacitance is to employ a pad electrode structure. However, the InP embedding layer is less of the ability to become highly resistive than is the InGaAs or IGaAsP contact layer. Thus, parasitic capacitances may exist between the proton implanted high resistance regions and the electrode.
It is therefore an object of the present invention to provide a semiconductor laser having a reduced capacitance, and a method of fabricating the laser for high speed operation.
According to a first aspect of the present invention, there is provided a semiconductor laser comprising a semiconductor substrate having a laser section and a modulator section, and a light waveguide having an active layer on the substrate and extending across the laser section and the modulator section from a high reflectivity end portion to a low reflectivity end portion. An embedding structure encloses the light waveguide. The embedding structure has a pair of high resistance regions one on each side of the light waveguide for confining electrons between the high resistance regions. First and second elongated contact regions are formed on the embedding structure corresponding respectively to the laser section and the modulator section. An insulating layer is provided for covering portions other than the first and second elongated contact regions. First and second electrodes are formed on the insulating layer for establishing ohmic contact with the first and second elongated contact regions, respectively.
According to a second aspect, the present invention provides a method of fabricating a seniconductor laser comprising the steps of (a) forming a light waveguide having an active layer on a semiconductor substrate having a laser section and a modulator section, the light waveguide extending across the laser section and the modulator section from a high reflectivity end portion to a low reflectivity end portion, (b) forming an embedding structure for enclosing the light waveguide, (c) forming first and second elongated contact regions on the embedding structure corresponding respectively to the laser section and the modulator section and producing first and second high resistance regions in the embedding structure on opposite sides of the light waveguide so that electrons are confined between the first and second high resistance regions, (d) forming an insulating layer for covering portions of the semiconductor laser other than the first and second elongated contact regions, and (e) forming first and second electrodes on the insulating layer for establishing ohmic contact with the first and second elongated contact regions, respectively.